1. Field of the Invention
The present invention relates to fiber optic transmission and more particularly to wavelength division multiplex transmission using dispersion shifted fiber as the line fiber.
2. Description of the Prior Art
The index profile of optical fibers is generally characterized as a function of the shape of the graph of the function that associates the radius of the fiber and the refractive index. It is conventional to plot on the abscissa axis the distance r to the center of the fiber and on the ordinate axis the difference between the refractive index and the refractive index of the cladding of the fiber. The expressions xe2x80x9cstep profilexe2x80x9d, xe2x80x9ctrapezium profilexe2x80x9d and xe2x80x9ctriangle profilexe2x80x9d are used to refer to graphs which respectively have step, trapezium and triangle shapes. These curves are generally representative of the theoretical profile or set point profile of the fiber and fiber fabrication constraints can yield a substantially different profile.
To use a fiber in a transmission system, and in particular in a wavelength division multiplex transmission system, it is beneficial for the fiber to have a large effective surface area in the range of wavelengths of the multiplex. A large effective surface area limits the power density in the fiber, at total constant power, and limits or prevents undesirable non-linear effects.
For high bit rate systems, it is also beneficial for the fiber to assure monomode propagation of the channels of the multiplex. ITU-T Recommendation G.650 defines the in-cable cut-off wavelength. The theoretical cut-off wavelength of the fiber is generally several hundred nanometers greater than the in-cable cut-off wavelength. It appears that propagation in an optical fiber can be monomode, even if the theoretical cut-off wavelength is greater than the wavelength of the signals used: beyond a distance of a few meters or tens of meters, which is small in comparison with the propagation distances in fiber optic transmission systems, the secondary modes disappear because of excessive attenuation. Propagation in the transmission system is then monomode.
It is also important for the fiber to have as small a sensitivity as possible to curvature and microcurvature. The sensitivity to curvature is evaluated, as explained in ITU-T Recommendation G.650, by measuring the attenuation caused by winding 100 turns of a fiber around a 30 mm diameter spool. The sensitivity to microcurvature is measured in a manner that is well known in the art; as described hereinafter, it can be measured relative to a fiber such as the ASMF 200 fiber manufactured by the assignees of the applicants.
In new high bit rate wavelength division multiplex transmission networks it is advantageous to limit the chromatic dispersion slope in the range of wavelengths of the multiplex; the objective is to minimize distortion between channels of the multiplex during transmission.
Dispersion shifted fibers (DSF) are now commercially available. Their chromatic dispersion is substantially zero at the transmission wavelength at which they are used, which is generally different from the wavelength of 1.3 xcexcm at which the dispersion of the silica is substantially zero; this means that the non-zero chromatic dispersion of the silica is compensatedxe2x80x94whence the use of the term xe2x80x9cshiftedxe2x80x9dxe2x80x94by an increase in the index difference xcex94n between the fiber core and the optical cladding. This index difference shifts the wavelength for zero chromatic dispersion; it is obtained by introducing dopants into the perform during its fabrication, for example by an MCVD process well known in the art and not described in detail here. Non-zero dispersion shifted fibers (NZ-DSF) are dispersion shifted fibers which have non-zero chromatic dispersion at the wavelengths at which they are used. The non-zero chromatic dispersion limits non-linear effects in the fiber and in particular four-wave mixing between the channels of the multiplex. The problem with DSF, as explained in EP-A-0 859 247, is that the chromatic dispersion slope generally increases as the effective surface area increases.
EP-A-0 859 247 describes ring profile DSF and explains that for such fibers there is a range in which the effective surface area and the chromatic dispersion slope vary in different directions. The fibers referred to by way of example have a negative chromatic dispersion in the range xe2x88x924.5 ps/(nm.km) to xe2x88x921.0 ps/(nm.km). They have a cut-off wavelength greater than 1500 nm for a fiber length of 2 m. The above document specifies that the high cut-off wavelength is not a problem because the cut-off wavelength decreases with the propagation distance and monomode propagation is assured for transmission distances in the order of 1 000 km.
In the article xe2x80x9cPractically feasible dispersion flattened fibers produced by VAD technique,xe2x80x9d ECOC""98 (p 131-132), Y. Yokohama et al. propose to obtain an effective surface area in the order of 50 xcexcm2 and a chromatic dispersion slope in the order of 0.026 ps/(nm2.km) by pushing the cut-off wavelength beyond 1550 nm.
In the article xe2x80x9cMaximum effective area for non-zero dispersion-shifted fibers,xe2x80x9d OFC""98 ThK3, P. Nouchi proposes a comparative study of the maximum effective surface area for different fiber profiles as a function of the curvature losses for fixed values of dispersion and dispersion slope. The above article shows in particular that fibers with a coaxial ring profile and fibers with a coaxial profile have larger effective surface areas, all other things being equal. However, this type of profile is not easy to fabricate, in particular because the central part of the index profile has an index less than or equal to that of the cladding.
Lucent TrueWave/RS fiber has the following specifications:
wavelength xcexo: 1 468 nm,
chromatic dispersion slope at 1550 nm: 0.045 ps/(nm2.km),
chromatic dispersion at 1550 nm: 3.7 ps/(nm.km),
mode diameter at 1550 nm: 8.4 xcexcm,
effective surface area at 1550 nm: 55 xcexcm2.
Corning LEAF NZ-DSF has an effective surface area of 72 xcexcm2 at 1550 nm and a chromatic dispersion slope in the order of 0.08 ps/(nm2.km) to 0.09 ps/(nm2.km); the chromatic dispersion is cancelled out at around 1500 nm.
The invention proposes an optical fiber that can be used in a cable and which represents an advantageous compromise between the effective surface area and the chromatic dispersion slope, in particular by virtue of the chosen cut-off wavelength, and which is additionally easy to fabricate.
To be more precise, the invention consists in an in-cable monomode optical fiber having a step and ring index profile and, for a wavelength of 1550 nm:
an effective surface area greater than or equal to 60 xcexcm2,
a chromatic dispersion from 3 to 14 ps/(nm.km),
a chromatic dispersion slope from 0 to 0.1 ps/(nm2.km), and
a ratio between the effective surface area and the chromatic dispersion slope greater than 900 xcexcm2.nm2.km/ps.
The fiber of the invention preferably has a chromatic dispersion at 1550 nm from 5 ps/(nm.km) to 11 ps/(nm.km) and/or a chromatic dispersion slope less than 0.07 ps/(nm2.km).
The fiber of the invention preferably has a ratio between the effective surface area and the chromatic dispersion slope greater than or equal to 1 000 xcexcm2.nm2.km/ps. This ratio is preferably less than or equal to 5000 xcexcm2.nm2.km/ps or even 2 500 xcexcm2.nm2.km/ps.
The fiber of the invention preferably has a chromatic dispersion cancellation wavelength xcexo less than or equal to 1 480 nm.
In one embodiment of the invention the fiber has an effective surface area greater than or equal to 70 xcexcm2.
The fiber of the invention has curvature losses at 1550 nm less than or equal to 0.05 dB and preferably less than or equal to 0.005 dB for 100 turns of fiber wound with a radius of 30 mm. It can also have a sensitivity to microcurvature less than 1.2 and preferably less than 0.8.
The fiber preferably has a theoretical cut-off wavelength greater than 1550 nm and an in-cable cut-off wavelength less than 1 300 nm.
In one embodiment of the invention the fiber has an attenuation at 1550 nm less than or equal to 0.23 dB/km and a polarization modal dispersion less than or equal to 0.1 ps.kmxe2x88x920.5.
The difference between the index of the central part of the fiber (step) and the index of the cladding is advantageously from 5xc3x9710xe2x88x923to 9xc3x9710xe2x88x923 and preferably from 6xc3x9710xe2x88x923 to 7.5xc3x9710xe2x88x923; the difference between the index of the intermediate segment between the step and the ring and the index of the cladding is from xe2x88x924xc3x9710xe2x88x923 to 1xc3x9710xe2x88x923 and preferably from xe2x88x922.5xc3x9710xe2x88x923 to 5xc3x9710xe2x88x924; the difference between the index of the ring and the index of the cladding is from 5xc3x9710xe2x88x924 to 5xc3x9710xe2x88x923 and preferably from 1xc3x9710xe2x88x923 to 3.5xc3x9710xe2x88x923.
The ratio between the radius of the step and the outside radius of the ring is advantageously from 0.23 to 0.45 and the ratio between the inside radius of the ring and the outside radius of the ring is from 0.45 to 0.75 and preferably from 0.48 to 0.7.
The outside radius of the ring is advantageously from 7 xcexcm to 13 xcexcm and preferably from 7.5 xcexcm to 11.5 xcexcm.
The invention also proposes a wavelength division multiplex fiber optic transmission system comprising a fiber as defined hereinabove as line fiber. It can then further comprise dispersion compensating fiber.